In the field of air navigation, an aircraft trajectory comprises a horizontal dimension and a vertical dimension. The skeleton of the horizontal trajectory of an aircraft is called the route which consists of a sequence of flight plan points joined by horizontal segments or legs. Each of these horizontal segments is defined between two waypoints, the final waypoint of a segment also forming the initial waypoint of the following segment of the route. The waypoints may for example be defined by the location of radionavigation beacons, or by geographical coordinates.
Each of the waypoints of the route can be associated with an altitude constraint. These constraints can in particular be of the following types: “AT” indicates that the aircraft must overfly a navigation point at a precise altitude; “AT OR ABOVE” indicates that the aircraft must overfly a navigation point at an altitude at least equal to the given altitude; “AT OR BELOW” indicates that the aircraft must overfly a navigation point at an altitude at most equal to the given altitude; “WINDOW” indicates that the aircraft must overfly the navigation point at an altitude lying in a window lying between a minimum altitude and a maximum altitude. These constraints are termed pointlike, in that they apply only at a point of the trajectory. The set of these constraints constitutes the vertical flight plan. The series of vertical segments linking these vertical constraints is called the reference vertical profile. It is optimized according to the speed constraints of the flight plan and the flight strategy given by the pilot. The rejoining vertical profile is the profile which, starting from the aircraft, will rejoin the reference vertical profile (if the aircraft is not exactly on the latter).
The constraints restricting a vertical profile can also be of a distributed nature. A distributed constraint applies to a vertical flight segment or a sub-part of a flight segment. Such is for example the case for certain descent phases which are subject to constraints of VPPL type (the acronym standing for Vertical Path Performance Limit). The VPPL constraints are in particular formalized by RTCA standard (the acronym standing for Radio Technical Commission for Aeronautics) DO-236C, which defines a lower and upper altitude limit in which an aircraft can fly at any point whilst following a vertical profile.
The calculation of a vertical flight plan for the aircraft is constrained by these altitude constraints. A vertical profile comprises a series of vertical segments making it possible to rejoin altitude constraints at successive given distances from the aircraft. This vertical profile, coupled with the route of the aircraft, makes it possible to define a prediction of the horizontal and vertical position of the aircraft throughout its trajectory.
In the known systems of FMS type, for a given flight plan, the horizontal trajectory on the one hand and the vertical profile on the other hand are produced separately. Initially, a horizontal trajectory is determined on the basis of the horizontal flight plan and of the speeds and flight level of the associated vertical flight plan. Thereafter, a vertical profile is produced, on the basis of the complete vertical flight plan (constraints and presets in the vertical plane) and of the horizontal trajectory. At the output of the vertical profile, the FMS has at its disposal the forecasts of altitude, speed, time, fuel, etc. for each of the points of the flight plan. As the radii of curvature of the lateral trajectory are dependent on the altitude and the aircraft speed, an iteration is performed on the flight plan and the lateral trajectory so as to adjust the angles of curvature (turns), thereby making it possible to obtain a flyable trajectory. This lateral trajectory having been recalculated, a new vertical profile must be generated. Iterations take place until the algorithm converges. In a general manner, the construction of the horizontal trajectory makes it possible to satisfy the contingency constraints of the trajectory, whilst the construction of the vertical profile makes it possible to satisfy the constraints pertaining to the flight domain of the aircraft.
In an optimal manner, the aircraft operates according to a guidance mode called managed mode. In this guidance mode, the position of the aircraft is slaved to a horizontal trajectory and a reference vertical profile. In this mode, the aircraft is slaved to the route. Stated otherwise, guidance laws are applied to the aircraft so that it follows the route gradually.
However, the aircraft may sometimes deviate from the reference trajectory. For example, it may deviate from the reference trajectory if the air traffic control instructs it to do so for safety reasons. It may also be correctly slaved to its horizontal trajectory, but not to its vertical profile. Such is for example the case when an unanticipated tailwind deflects the aircraft slightly from its trajectory, and when the forecasts recalculated on the vertical flight segments initially calculated while taking account of this situation no longer comply with the vertical constraints to which the trajectory of the aircraft is subject. Such is also the case when a modification of the horizontal flight plan arises, and when the recalculated forecasts no longer correspond to the constraints associated with the aircraft's various navigation points. Such is again the case when a manual lateral guidance instruction disengages the mode for automatic following of the vertical profile and causes the aircraft to diverge from the latter. Finally, in an upgrade of the guidance function, it may no longer be slaved to its lateral trajectory, but continue to be slaved to the vertical profile recalculated on the basis of the lateral rejoining trajectory, as disclosed for example in patent FR 2924505.
In this case, the predicted vertical profile of the aircraft may no longer comply with certain altitude constraints, and must therefore be adapted.
A first method consists in recalculating a complete vertical profile. This method is for example described in document FR2983594 and exhibits the drawback of being lengthy to implement. Moreover, a complete recalculation of the vertical profile may significantly modify the vertical profile of the aircraft (for example, modify the orders of the flight phases, the cruise altitude etc.), these modifications possibly being difficult for the air traffic control to manage when the air traffic is dense, and disturbing for the crew of an aircraft with the heavily modified vertical profile.
The object of the invention is to exceed the limits of the prior art, by proposing a scheme for automatically adapting a vertical trajectory, allowing an aircraft, when its vertical trajectory no longer complies with the vertical constraints restricting them, to adapt the trajectory so as to rejoin a reference vertical profile as quickly as possible, while complying with the flight domain of the aircraft.